Atrial fibrillation (AF), the most common sustained cardiac arrhythmia in Veterans, is associated with increased risk for stroke, heart failure, and death. The number of Americans affected by AF is expected to surge to approximately 16 million by the year 2050. Although a causal relationship between obesity and AF was recently established, the underlying pathophysiological mechanisms and their impact on response to antiarrhythmic drug (AAD) therapy remain unclear. Emerging evidence supports reduced cardiac Na+ channel expression as one potential contributing mechanism. Since Class I AADs, which block the cardiac Na+ channel (Nav1.5), are commonly used to treat AF, the overarching goal of this proposal is to elucidate the underlying electrophysiologic (EP) and molecular mechanisms by which obesity increases risk of AF and modulates response to Na+ channel blockers in diet-induced obese (DIO) mice. Specific Aim 1 will test the hypothesis that obesity-induced AF is associated with increased oxidative stress in DIO mice and this effect is in part mediated by modulating the cardiac Na+ channel. We will measure biomarkers of atrial (4-hydroxynonenal, 4-HNE; nitrated proteins, YNO2) and systemic (F2-isoprostanes, IsoPs) oxidative stress in DIO mice and compare them with lean controls. This aim builds on our previous work showing that SCN5A loss-of- function mutations increase AF risk by reducing Nav1.5 expression and current (INa), pilot data showing that DIO mice are more prone to AF than lean controls and this risk is mediated in part by downregulation of Nav1.5 and increased oxidative stress. Despite recent advances in catheter-based therapies, AADs continue to be commonly used to treat symptomatic AF. However, response in an individual patient is highly variable and membrane-active drugs are associated with serious toxicities. Thus, a major knowledge gap is predicting which patients with AF are most likely to respond to AADs. Our pilot data, generated in the JBVA/UIC AF Registry, not only shows that obesity modulates response to antiarrhythmic therapy but that there is a differential response to Na+ channel versus K+ channel blocker AADs. This raises the hypothesis to be tested in Specific Aim 2 that flecainide (Na+ channel blocker) is inferior to sotalol (K+ channel blocker) in treating AF in DIO mice. Obese mice will undergo rapid transesophageal atrial pacing to induce AF and then be treated with AADs acutely or chronically with AF burden as the primary outcome. This aim builds on our clinical and animal data that shows reduced efficacy of flecainide in treatment of AF in obese patients and DIO mice respectively. Our studies have shown that obesity-mediated AF is associated with increased oxidative stress and this is mediated in part by modulation of the cardiac Na+ channel. Specific Aim 3 will define the underlying molecular mechanisms by which obesity increases risk of AF in DIO mice by: i) identifying the sources and specific pathways of ROS production; ii) determining if Nav1.5 is directly targeted by increased oxidative stress; and iii) evaluating how oxidative stress impacts the expression and activity of the cardiac Na+ channel. This aim builds on our earlier studies and pilot data where we show significant reduction in AF burden in DIO mice treated with a mitochondria-targeted antioxidant (MitoTEMPO). Direct impact of the proposed studies will elucidate the underlying EP and molecular mechanisms by which obesity increases risk of AF; identify novel atrial biomarkers of oxidative stress that will translate to patients; uncover specific pathways of ROS production for therapeutic targeting; and test the hypothesis that results from mouse models can be translated into better antiarrhythmic therapy in obese patients with AF. 1